In the manufacture of microelectronic devices, various microelectronic components are electrically joined together into circuits using metal alloy solders. Such solders are generally alloys of lead (Pb) and tin (Sn), but can comprise other metal components. The metal alloy solder is generally optimized for bulk properties that control the thermal behavior of the solder. For example, the melting temperature of Pb/Sn solder is controlled by adjusting the relative amounts of tin and lead in the solder. The minimum melting temperature of a Pb/Sn alloy is achieved when the ratio by weight of Pb to Sn in the solder is 37:63. This is called the eutectic composition.
For some applications, it is desirable to use bulk solder alloys at other than the eutectic composition. For example, Controlled Collapse Chip Connection (C4) solder balls on Integrated Circuit (IC) chips, which are used to join the chips to chip carriers, are typically Pb-rich in the range of 90% to 97% Pb. Because Pb-enriched solders have higher melting temperatures than solders at or nearer the eutectic composition, the solder joint between the chip and the chip carrier maintains its shape throughout subsequent assembly of the chip carrier (with chip attached) to another electrical component such as a printed circuit board. The Pb-enriched solders also typically require higher temperatures, however, to fuse them to metal surfaces such as copper pads on chip carriers. Thus, organic laminate chip carriers having copper joining pads may be exposed to undesirable high temperatures during joining processes.
Such Pb-enriched solders may also be poorly suited for surface treatment to promote fluxless soldering. One fluxless soldering process is known as Plasma-Assisted Dry Soldering (PADS). In the PADS process, solder is treated in a plasma environment containing fluorine atoms to produce a surface that does not require the use of organic solder flux to remove metal oxides before electrically joining a chip to a chip carrier.
All metal solder surfaces typically oxidize when exposed to ambient oxygen in the air, forming a metal oxide layer on the surface of the solder. Flux is an organic acid typically externally applied to a solder surface before a fusing step. The flux reacts with the metal oxide layer when heated, thus converting the metal oxides to salts and exposing the underlying solder metal so that it may be fused to another metal surface. Fluorine treatment is thought to react with the metal oxides to form hydrolytically unstable metal oxyfluoride compounds on the solder surface. Upon heating, the metal oxyfluoride compounds hydrolyze, most likely to hydrogen fluoride that converts the oxides to salts in the same way that the flux does.
For Pb/Sn solders, it has been demonstrated that fluorine atoms react preferentially with the Sn oxides. Thus, fluorine treatment is not as effective for solder compositions that are "too rich" in Pb, such as commonly used 97:3 Pb:Sn solder. So, for instance, although fluxless soldering of 90:10 Pb:Sn alloy solder has been achieved previously after exposure of the solder to fluorine atoms, it has only been accomplished at temperatures above 300.degree. C., beyond the desired temperature range for the organic laminate chip carriers.
Thus, a need remains for a process that enables fluxless soldering of Pb-enriched solders at temperatures less than 300.degree. C.